The Brown Trout (Salmo trutta morpha fario and S. trutta morpha lacustris) and the Sea Trout (S. trutta morpha trutta) are fish of the same species.

They are distinguished chiefly by the fact that the brown trout is largely a freshwater fish, while the sea trout shows anadromous reproduction, migrating to the oceans for much of its life and returning to freshwater only to spawn. Sea trout in the UK and Ireland have many regional names including sewin (Wales), finnock (Scotland), peal (West Country), mort (North West England) and white trout (Ireland).

The lacustrine morph of brown trout is most usually potamodromous, migrating from lakes into rivers or streams to spawn, although there is some evidence of stocks that spawn on wind-swept shorelines of lakes. S. trutta morpha fario form stream-resident populations, typically in alpine streams but sometimes in larger rivers. There is evidence that anadromous and non-anadromous morphs coexisting in the same river can be genetically identical. In common usage, the name "brown trout" is often applied indiscriminately to the various morphs.

Range

The brown trout is normally considered to be native to Europe and Asia, but the natural distribution of the migratory forms may be, in fact, circumpolar. There are also landlocked populations far from the oceans, for example in Greece and Estonia.

Conservation Status

The fish is not considered to be endangered although, in some cases, individual stocks are under various degrees of stress mainly through habitat degradation, overharvest and artificial propagation leading to introgression. Increased frequency of excessively warm water temperatures in high summer, attributed to global warming, causes a reduction in dissolved oxygen levels which can cause 'summer kills' of local populations if temperatures remain high for sufficient duration and deeper/cooler or fast, turbulent more oxygenated water is not accessible to the fish. This phenomenon can be further exacerbated by eutrophication of rivers due to pollution - often from the use of agricultural fertilizers within the drainage basin.

Overfishing is a problem where anglers fail to identify and return mature female fish into the lake or stream. Each large female removed can result in thousands fewer eggs released back into the system when the remaining fish spawn.

Another threat is other introduced organisms. For example in Canada's Bow River, a non-native algae Didymosphenia geminata - common name 'Rock Snot' (due to appearance) - has resulted in reduced circulation of water amongst the substrate of the river bed in affected areas. This in turn can greatly reduce the number of trout eggs which survive to hatch. Over time, this leads to reduction of the population of adult fish in the areas affected by the algae; forming a circle of decline. Rock Snot is believed to have spread accidentally on the soles of the footwear of visitors from areas where the algae is native. The wide variety of issues that adversely affect brown trout throughout its range, do not exclusively affect brown trout, but affect many or all species within a water body, thus altering the ecosystem in which the trout reside.

In small streams, brown trout are important predators of macro-invertebrates, and declining brown trout populations in these specific areas would affect the entire aquatic food web. S. trutta morpha fario prefers cold (though in comparison with other trout, this species has a somewhat higher temperature preference of about 60-65 °F, or 15.5-18.3 °C), well-oxygenated upland waters, especially large streams in mountainous areas.

Cover or structure is important to trout, and they are more likely to be found near submerged rocks, undercut banks, and overhanging vegetation. Structure provides protection from predators, bright sunlight and associated high water temperatures. Access to deep water for protection in winter freezes, or fast water for protection from low oxygen levels in summer are also ideal.

Characteristics

The brown trout is a medium-sized fish, growing to 20 kg or more in some localities although in many smaller rivers a mature weight of 1 kg (2 lb) or less is common. The spawning behaviour of brown trout is similar to that of the closely related Atlantic salmon. A typical female produces about 2,000 eggs per kilogram (900 eggs per pound) of body weight at spawning. On Sept. 11, 2009, a 41.45 lb (18.80 kg) Brown trout was caught by Tom Healy in the Manistee river system in Michigan, setting a new state record and possibly, a new world record for Brown trout. As of late December 2009, the fish captured by Mr. Healy was confirmed by both the International Game Fish Association and the Fresh Water Fishing Hall of Fame, as the new all tackle world record for the species. This fish now supplants the former world record from the Little Red River in Arkansas.

Brown trout can live to ages of 20 years. But as with the Atlantic salmon, there is a high proportion of death of anadromous males after spawning and probably fewer than 20% of anadromous female kelts recover from spawning. The migratory forms grow to significantly larger sizes for their age due to abundant forage fish in the waters they spend most of their lives. Brown trout are active both by day and by night and are opportunistic feeders. While in fresh water, the diet will frequently include invertebrates from the streambed, other fish, frogs, mice, birds, and insects flying near the water's surface. The high dietary reliance upon insect larvae, pupae, nymphs and adults is what allows trout to be a favoured target for fly fishing. Sea trout are especially fished for at night using wet flies. Brown trout can be caught with lures such as spoons, spinners, jigs, plugs, plastic worm imitations, and live or dead baitfish. Freshwater brown trout range in colour from largely silver with relatively few spots and a white belly, to the more well known brassy brown cast fading to creamy white on the fish's belly, with medium-sized spots surrounded by lighter haloes. The more silver forms can be mistaken for rainbow trout. Regional variants include the so-called "Loch Leven" trout, distinguished by larger fins, a slimmer body, and heavy black spotting, but lacking red spots. The continental European strain features a lighter golden cast with some red spotting and fewer dark spots. It is important to remember that both strains can show considerable individual variation from this general description. Early stocking efforts in the United States used fish taken from Scotland and Germany. The Loch Leven strain is more often found in the western United States, while the "German brown" is found more toward the Midwest and East.

Brown trout rarely form hybrids, almost invariably infertile, with other species. One such example is the tiger trout, a hybrid with the brook trout.

Diet

Young brown trout feed on insects and other invertebrates such as shrimp, flies, caddis, stonefly, mayfly, etc. Both larvae and adults are taken and the fish will eat whatever local insect life is abundant at the time. Larger fish are active predators of fish including young brown trout, suckers, sculpins, shad, whitefish and rainbow trout. Larger brown trout will also feed on small terrestrial animals that fall into the water such as baby birds falling from overhanging nests, or even swimming mice/voles. Brown trout sometimes do not actively feed until the late afternoon or early evening but when the weather is cool they will feed during the day as well. The largest browns feed under cover of darkness. Brown trout can be caught with artificial flies, jigs, plastic worm imitations, spinners and other lures.

Stocking, Farming and Non-Native Brown Trout

The species has been widely introduced for purposes of sport into North America, South America, Australia, New Zealand and many other countries including Bhutan where they are the focus of a specialised fly fishery. First planting in the United States occurred April 11, 1884 into the Baldwin River, one mile east of Baldwin, MI. Brown trout have had serious negative impacts on upland native fish species in some of the countries where they have been introduced, particularly Australia. Because of the trout's importance as a food and game fish, it has been artificially propagated and stocked in many places in its range, and fully natural populations (uncontaminated by allopatric genomes) probably exist only in isolated places, for example in Corsica or in high alpine valleys on the European mainland.

Farming of brown trout has included the production of infertile triploid fish by increasing the water temperature just after fertilisation of eggs, or more reliably by a process known as pressure shocking. Triploids are favoured by anglers because they grow faster and larger than diploid trout. Proponents of the stocking of triploids argue that, because they are infertile, they can be introduced into an environment that contains wild brown trout without the negative effects of cross-breeding. However, it is possible that stocking triploids may damage wild stocks in other ways. Triploids certainly compete with diploid fish for food, space and other resources. They could also be more aggressive than diploid fish and they may disturb spawning behaviour.

Scottish and Irish sea trout populations in recent years have seriously declined due to infestation by sea lice from salmon farms